1. Field of the Invention
The present invention relates in general to an improved main shaft for a coal pulverizer, and more particularly to an improved main shaft for use in B&W Type E and EL pulverizers
2. Description of the Related Art
Coal pulverizers are widely used in the electric utility industry for the preparation of crushed coal for efficient combustion. A dominant type of pulverizer for this service is the vertical spindle type. For many years this type featured an integral speed reducer. The output shaft for the speed reducer has a closely fitted mechanical joint with the grinding table and this joint is subjected to severe stresses from driving forces and from impact, shock, and eccentric loading from the grinding process.
FIG. 1 shows a cross section of a B&W type EL pulverizer generally depicted as numeral 2. These devices are used to crush coal for burning in a furnace or boiler. This type of pulverizer has a stationary top ring 4, one rotating bottom ring 6, and one set of balls 8 that comprise the grinding elements. The pressure required for efficient grinding is obtained from externally adjustable dual purpose springs 10. The bottom ring 6 is driven by the yoke 12 which is attached to a vertical main shaft assembly 14 of the pulverizer. The top ring 4 is held stationary by the dual purpose springs 10. Raw coal is fed into the grinding zone where it mixes with partially ground coal that forms a circulating load. Pulverizer air causes the coal to circulate through the grinding elements where some of it is pulverized in each pass through the row of balls 8. As the coal becomes fine enough to be picked up by the air it is carried to the classifier where coal of a desired fines is separated from the stream entering the classifier and is carried out with the air. Oversized material is returned to the grinding zone.
The pulverizer is driven by spiral bevel gears positioned on horizontal pinion shaft 16 and vertical main shaft 14 located in the base. Both the vertical main shaft 14 and the horizontal pinion shaft 16 are mounted in roller bearings. Forced lubrication is provided for the entire gear drive by an oil pump submerged in the oil reservoir and gear-driven from the pinion shaft.
Currently, there is some concern as to main shaft 14 failure. It is believed that the failures occur because of bending fatigue originating at fretted surfaces in the lower contact land with the yoke bushing bore. Fretting damage, sometimes referred to as fretting corrosion, is a condition of surface deterioration brought on by very small relative movements between bodies in contact. The fit between the yoke bushing and main shaft is an interference type fit. This type fit generates a stress concentration or multiplier. The pulverizer design generates cyclic or alternating type bending loads in the top end of the main shaft. Because the loads are cyclic, rubbing or fretting corrosion will occur. Also of concern is fatigue failure when stress concentration, cyclic loading and fretting corrosion are combined. Like fretting, fatigue has a definite set of characteristics which combine to identify this failure phenomenon. Pulverizer vibration usually results in high shaft stress levels and may have a role in main shaft failures. Vibration may be caused by abnormal grinding element wear such as out-of-round wear of balls or rings. Pulverizer vibration also will occur if proper air/fuel regulation for the burners is not provided.
Because of the foregoing, there have been many attempts to correct main shaft failure frequency such as employing an anti-seize compound at the taper joint, using a bushing with a undercut center portion, using full contact bushings with no undercut center portion, shot peening, and nitriding as a surface hardening process. Remedial efforts notwithstanding, even carefully fitted taper joints, when subjected to cyclic bending forces often exhibit vulnerability to fatigue failure of shafts because of fretting and strain produced within the assembly.
There still exists a need for an improved main shaft assembly for these types of pulverizers, one that will provide improved fretting resistance to reduce shaft failure due to fretting-induced bending fatigue on ball-race coal pulverizers.